1. Field of the Invention
This invention relates to radial power divider/combiners for use in solid-state power amplifiers (SSPAs), and more particularly to a multi-layer topology that realizes the cost benefits of planar fabrication without compromising the isolation characteristics of a Wilkinson divider/combiner for N-way devices where N is greater than two.
2. Description of the Related Art
Solid state power amplifier (SSPAs) modules are comprised of N identical amplifier devices that are combined into a single amplifier structure using a passive divider/combiner. SSPAs have a variety of uses. For examples, SSPAs may be used in satellites to provide transmit power levels sufficient for reception at ground-based receivers, or to perform the necessary amplification for signals transmitted to other satellites in a crosslink application. SSPAs are also suitable for ground-based RF applications requiring high output power such as cellular base stations. SSPAs are typically used for amplification from L-band to Ka-band (with future applications at even higher frequencies) spanning wavelength range of approximately 30 to 0.1 cm (approximately 1 GHz to 300 GHz).
Typical millimeter wave SSPAs achieve signal output levels of more than 10 watts. A single amplifier chip cannot achieve this level of power without incurring excessive size and power consumption (low efficiency). As shown in FIG. 1, an SSPA 10 uses a splitting and combining architecture in which the signal is divided into a number of individual parts and individually amplified. A 1:N power divider 12 splits input signal 14 into individual signals 16. Each signal is amplified by a respective amplifier chip 18 such as a GaAs pHEMT or GaN HEMT technology device. The output signals 20 of the amplifiers are then combined coherently via an N:1 power combiner 22 into a single amplified output signal 24 that achieves the desired overall signal power level. To maintain amplifier performance it is important that the paths through the power combiner are low loss, well isolated and have minimum phase errors.
Wilkinson developed the first isolated power divider/combiner 30 in 1959 as shown in FIGS. 2a and 2b. Wilkinson's N-way divider uses quarter-wave sections 32 of transmission lines for each arm that are isolated from each other by a star resistor network 34. The star resistor includes N resistors 36 connected at a common junction 38 (not ground). Each resistor 36 is connected to one of the quarter-wave sections 32 at a port 40 to external loads 42. These “loads” are comprised of the inputs or outputs of the amplifiers in an SSPA, depending on whether the splitter is used as a combiner or divider. The other ends of the quarter-wave sections 34 are joined at a common port 44 to an external load 46. In the case of a divider, this “load” would be the signal generator. Another quarter-wave section or cascade of sections (not shown) may be coupled to the common port to extend the bandwidth. Because sections 32 are ‘quarter-wave’ they function as an impedance matching transformer. Consequently the impedance seen looking into any of the individual ports 40 or common port 44 is Z0, the desired system impedance (typically 50 ohms). Impedance matching is important and common practice to eliminate mismatches that could cause gain ripples or reduced power in an SSPA combiner due to load-pull effects.
An N-way power divider/combiner works as follows. As a power divider, a signal enters the common port 1 and splits into equal-amplitude, equal-phase output signals at ports 2, 3, . . . N+1. Because each end of the isolation resistor 36 between any two ports 40 is at the same potential, no current flows through the resistor and therefore the resistor is decoupled from the input and dissipates none of the split signal power. As a power combiner, one must consider that equal amplitude/phase signals enter ports 2 through N+1 simultaneously. Again, each end of any isolation resistor is at the same potential and dissipates none of the combined signal power. To understand the port isolation that the resistor network provides, consider the case where a single signal is made to enter one of ports 2 through N+1. A fraction of its power (ideally, 1/N) will appear at Port 1, and the remainder of the signal is fully dissipated in the resistor network (if perfect isolation is provided), with none of the signal appearing at the other ports.
The N-way Wilkinson power divider can provide (ideally) perfect isolation at the center frequency, and adequate isolation (20 dB or more but this figure of merit is arbitrary and depends on design circumstances) over a substantial fractional bandwidth: isolation bandwidth can be increased by cascading multiple quarter-wavelength sections and adding additional isolation networks (star resistors for N>2).
In theory, Wilkinson's design can provide near perfect isolation and wide bandwidth. However, perfect isolation is never attained because electrically ideal resistors are not possible. These resistors are preferably as short as possible to minimize the phase angle that separates any two paths. However, even the smallest resistor induces a finite phase that limits isolation of the N ports and corrupts port impedance matching. Two resistors coupled in series each having an electrical length of λc/20 produces a path length of λc/10, which corresponds to a transmission phase angle of +36 degrees. To dissipate power caused by slightly mismatched amplifiers in an SSPA or a failure of one of its amplifiers the isolation resistor of the combiner network must be large enough to dissipate the worst-case heat load, which in turn induces a larger transmission phase. Maintaining symmetry of the isolation network and a near zero transmission phase angle is important to avoid degradation of RF performance.
Although two-way power divider/combiners are manufactured using planar technology, a significant limitation of a Wilkinson power divider/combiner is that it cannot be designed to take advantage of the lower production costs and other benefits of planar metallization technology for N greater than two. As shown in FIG. 2a, the star resistor 34 is placed at the end of cylinder and the quarter-wave sections 32 are placed longitudinally along the cylinder. This configuration preserves the isolation network but is expensive to manufacture and difficult to integrate into an SSPA. Planar metallization technology has not generally been applied to the N-way Wilkinson combiner because of topological problems that arise in physically locating the isolation resistors 36 so that they can be conveniently assembled but yet can properly dissipate incident power due to imbalances in the amplifiers or upon failure of the amplifier chips. Inadequate capacity or the isolating resistors to dissipate power causes unpredictable effects in the power output level of the composite amplifier upon failure of an elemental amplifier, or catastrophic failure of the entire SSPA.
For higher order, N>2, power divider/combiners the isolation network is either compromised for a planar layout as shown in FIGS. 3 and 4 or corporate strictures of 2:1 devices are employed as shown in FIG. 5. As shown in FIG. 3, a three-way Wilkinson power divider/combiner 50 is implemented in a planar topology by using a two-dimensional approximation of the Wilkinson device shown in FIGS. 2a and 2b. This is an N=3, two-section design where the RF passes through two quarterwave (90 degree) sections in cascade. In this case, one of the three isolation resistors is deleted from the layout and a “fork” arrangement is the result. The penalty that is paid for the compromised planar layout is reduced isolation and bandwidth. It is difficult to achieve 20 dB isolation between the opposite arms of this type of network over even a 10% bandwidth. As shown in FIG. 4, a 12-way planar radial combiner 60 provides isolation resistors 62 between adjacent paths. Isolation between the adjacent paths is high but isolation between non-adjacent paths is sacrificed. As shown in FIG. 5, an eight-way power divider/combiner 70 is implemented using a corporate structure of three stages of 2:1 divider/combiners 72 cascaded together. The penalty for this approach is increased RF losses, not just in the cascaded divider/combiner elements but in the interconnecting lines that are used to connect the stages. Additionally, the value of N is restricted to binary solutions such as N=2, N=4, N=8 and N=16. The unit cell 2:1 divider in this example is a three-section design where the RF passes through ¾ of a wavelength. The phase relationships between ports 2 through 9 are not maintained (the outside four paths are longer than the inside four paths), therefore it is not suitable for an SSPA. Some of the split signals must travel a path length of more than three wavelengths.